I'm using KDE5 at the moment, and it's essentially problem-free. It's got more features than I know what to do with, but this somehow doesn't detract from it but rather enhances it.
KDE5 never seems to be the default choice of desktop environment for distros like Manjaro, but you're unlikely to go wrong picking it as your default option, if you're given that option.
I use it on FreeBSD myself. It's an amazing desktop. I don't like gnome at all. It uses too much screen space (huge touch style window decorations) and doesn't have enough customisation for me. But KDE is perfect.
> openSUSE installer provides three officially supported desktop options. There is no default choice
Coming at this from another angle, their installer isn't super opinionated. The KDE radio button is at the top of the list on the desktop selection page.
Does that make it the default choice? I guess it does by some definition.
KDE5 never seems to be the default choice of desktop environment for distros like Manjaro, but you're unlikely to go wrong picking it as your default option, if you're given that option.